Year of Publication

2012

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture

Department

Plant Pathology

First Advisor

Dr. Pradeep Kachroo

Abstract

Oleic acid (18:1), a monounsaturated fatty acid (FA), is synthesized upon desaturation of stearic acid (18:0) and this reaction is catalyzed by the plastidal enzyme stearoyl-acyl carrier protein-desaturase (SACPD). A mutation in the SSI2/FAB2 encoded SACPD lowers 18:1 levels, which correlates with induction of various resistance (R) genes and increased resistance to pathogens. Genetic and molecular studies have identified several suppressors of ssi2 which restore altered defense signaling either by normalizing 18:1 levels or by affecting function(s) of a downstream component. Characterization of one such ssi2 suppressor mutant showed that it is required downstream of low 18:1-mediated constitutive signaling and partially restores altered defense signaling in the ssi2 mutant. Molecular and genetic studies showed that the second site mutation was in the Nitric Oxide Associated (NOA) 1 gene, which is thought to participate in NO biosynthesis. Consistent with this result, ssi2 plants accumulated high levels of NO and showed an altered transcriptional profile of NO-responsive genes. Interestingly, the partial defense phenotypes observed in ssi2 noa1 plants were completely restored by an additional mutation in either of the two nitrate reductases NIA1 or NIA2. This suggested that NOA1 and NIA proteins participated in NO biosynthesis in an additive manner. Biochemical studies showed that 18:1 physically bound NOA1, in turn leading to its degradation in a protease-dependent manner. In concurrence, overexpression of NOA1 did not promote NO-derived defense signaling in wild-type plants unless 18:1 levels were lowered. Subcellular localization showed that NOA1 and the 18:1-synthesizing SSI2 were present in close proximity within the nucleoids of chloroplasts. Indeed, pathogen- or low 18:1- induced accumulation of NO was primarily detected in the chloroplasts and their nucleoids. Together, these data suggested that 18:1 levels regulate NO synthesis and thereby NO-mediated retrograde signaling between the nucleoids and the nucleus. Since cellular pools of glycerol-3-phosphate (G3P) regulate 18:1 levels, I next analyzed the relationship between G3P and 18:1. Interestingly, unlike 18:1, an increased G3P pool was associated with enhanced systemic immunity in Arabidopsis. This was consistent with G3P-mediated transcriptional reprogramming in the distal tissues. To determine mechanism(s) underlying G3P-conferred systemic immunity, I analyzed the interaction between G3P and a lipid transfer protein (LTP), DIR1. In addition, I monitored localization of DIR1 in both Arabidopsis as well as tobacco. Contrary to its predicted apoplastic localization, DIR1 localized to endoplasmic reticulum and plasmodesmata. The symplastic localization of DIR1 was confirmed using several different assays, including co-localization with plasmodesmatal-localizing protein, plasmolysis and protoplast-based assays. Translocation assays showed that G3P increased DIR1 levels and translocated DIR1 to distal tissues. Together, these results showed that G3P and DIR1 are present in the symplast and their coordinated transport into distal tissues is likely essential for systemic immunity.

In conclusion, this work showed that low 18:1-mediated signaling is mediated via NO, synthesis of which is likely initiated in the plastidal nucleoids. In addition, my work shows that G3P functions as an independent signal during systemic signaling by mediating translocation of the lipid transfer protein, DIR1.

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